A photolithography system includes an illumination source which directs radiation such as light through a reticle or photomask onto a photosensitive material, such as photoresist, coated on a substrate, such as a semiconductor wafer, on which a pattern, such as a circuit pattern, is to be formed. As circuits have become more highly integrated, devices have become extremely small. Therefore, the patterns formed on the circuit wafer and, as a result, the corresponding patterns on photomasks or reticles used to make the circuit patterns, have also become extremely small. In order to produce such small patterns, the photolithography system must have extremely high resolution.
To improve feature resolution in a photolithography system, it has become important that the optics in the system gather as much light as possible from the source. With standard normal or vertical illumination in which the beam of radiation is incident on the photomask perpendicular or normal to the photomask, much of the light that is gathered by the exposure system and reaches the substrate is 0th-order light. To improve resolution, it is important to gather and direct higher orders of light generated by diffraction by the photomask pattern, and to direct these higher orders of light onto the photosensitive coating. Specifically, it is desirable to collect first-order light generated by diffraction by the photomask pattern and to illuminate the coating with the first-order light.
Off-axis illumination (OAI) by the source is one way to collect first-order light and direct it onto the wafer being exposed. OAI refers to an illumination shape that significantly reduces or eliminates the on-axis or vertical component of the illumination, that is, the light striking the photomask at near-normal incidence. By tilting the illumination away from normal incidence, the diffraction pattern of the mask is shifted within the objective lens of the exposure system, thus allowing more of the first-order light from the photomask to be directed onto the wafer.
FIG. 1 is a schematic functional diagram of an illumination system used in photolithography exposure processes. Referring to FIG. 1, the system 10 includes an illumination source 12, such as a high-power excimer laser, and conditioning optics for illuminating a photomask or reticle 26. The illuminated region of the mask 26 is imaged by the projection system onto the wafer (not shown). The laser source 12 directs a beam of illuminating radiation such as light 14 onto a beam shaping component 16. The beam shaper 16 is used to control the beam 14 to create an illumination beam 21 of a desired shape. In one particular configuration, the beam shaper 16 includes a deflective optical element (DOE) 18 and a lens 20. The shaped beam 21 from the beam shaper 16 is directed onto a uniformizer 22, which makes the light uniform throughout its entire shape. The condenser lens 24 collects the light and focuses it onto the reticle or photomask 26. The illumination shown in FIG. 1 is normal-incidence or vertical, on-axis illumination.
FIG. 2 contains a schematic diagram of a portion of a lithographic exposure system illustrating principles of lithographic exposure. Referring to FIG. 2, light is vertically incident to the photomask 26 on the back or bottom side of which are formed opaque light-screening patterns. After passing through and being diffracted by the fine mask patterns, the light is collected and transmitted by a projection lens 29 to a wafer 27 at which images of the patterns on the photomask are formed.
When such vertically incident light passes through the slits between the fine patterns on the photomask 26, it is diffracted and separated largely into 0th-order light and 1st-order light under the influence of the narrow width of the patterns. Almost all light rays coming from the large slit width patterns are 0th-order or ±1st-order with a diffraction angle of θ1. Information about the mask is contained in 1st-order light. To resolve patterns, information about patterns should fall on and be collected by the lens 29. Referring to FIG. 2, under vertical illumination, much of the ±1st-order light is not collected by the lens 29. This can result in reduced resolution of the system. If more of the ±1st-order light were to fall on the lens 29, resolution would be improved.
FIG. 3 contains a schematic diagram further illustrating this diffraction effect in a photolithography exposure system 10. Referring to FIG. 3, the wafer 27 is mounted on a stage 41 in the system 10. The photomask 26 includes a pattern of opaque lines 40 separated by slits or spaces. Light is vertically incident to the photomask 26 on which the patterns 40 are formed. After passing through and being diffracted by the fine patterns 40, the light is transmitted from the projection lens 29 to the wafer 27 mounted on the stage 41 to form images of the pattern on the wafer 27.
As described above in connection with FIG. 2, when such vertically incident light passes through the slits between the patterns, it is diffracted and separated largely into 0th-order light and ±1st-order light under the influence of the narrow width of the patterns. Almost all light rays coming from the comparatively larger slit width patterns are 0th-order or 1st order with a diffraction angle of θ1. The light rays coming from finer slit width patterns are mostly ±1st-order with a diffraction angle of θ2, which is greater than θ1, as illustrated in FIG. 3. That is, as the feature size and design pitch become smaller, the deflection angle of the ±1st-order light increases. The ±1st-order light has relatively high intensity, and a difference in optical path between the 0th-order light and the ±1st-order light is created by the radius of curvature and focus distance of the projection lens 29, thus degrading image contrast and, therefore, resolution.
As the size of features on the photomask approaches or drops below the wavelength of the exposure illumination, diffraction effects become more pronounced. This can cause the first order of the diffracted image to fall outside the projection lens 29, thereby causing imaging problems, because both the zero and first diffraction orders must fall on the projection lens in order to properly resolve the image from the photomask.
FIG. 4 contains a schematic diagram illustrating the effect of off-axis illumination (OAI) on the diffraction effect in a photolithography exposure system 110. OAI refers to any illumination shape that significantly reduces or eliminates the on-axis component of illumination, that is, the light striking the photomask at normal or near-normal incidence. By tilting the illumination away from normal incidence, the diffraction pattern of the mask is shifted within the objective lens 29. More of the ±1st-order light is collected and directed onto the photomask, and resolution is improved.
OAI improves resolution by illuminating the photomask with light off the optical axis of the stepper lens of the exposure system 110. The interaction of light at an angle, falling on the photomask structures that act as diffraction gratings, can improve the contrast of the image by transmitting more of the diffracted orders through the projection lens 29. That is, the deflection angle θ2 of the ±1st-order light is reduced, such that more of the ±1st-order light is used in imaging the photomask pattern on the wafer 27.
OAI is one of several major resolution enhancement approaches that have enabled optical lithography to advance practical resolution limits far beyond what was once thought possible. Other approaches include phase-shifting masks and optical proximity corrections. In order to effectively use OAI, the shape and size of the illumination must be optimized for the specific mask pattern being printed.
OAI involves symmetrically illuminating the photomask from more than one direction off the optical axis of the lithography system. The zero and first diffraction orders from each illumination point reach the lens with the symmetrical arrangement compensating for any shift in the image. Also, imaging errors in the horizontal-to-vertical line performance or aberrations in the projection lens may require asymmetrical illumination in order to compensate for the error, including adjusting the illumination balance throughout the illumination pattern, e.g., causing one pole to be of higher intensity than another, and/or adjusting the shape of the illumination pattern, e.g., the horizontal of the illumination pattern may be elongated to be larger than the vertical of the illumination pattern.
Various illumination modes may be utilized, depending on the pattern being formed and other factors. These modes include annular illumination and multipole illumination. Annular illumination may include a single ring, concentric rings, etc. Multipole illumination modes may include illumination patterns having any number of poles, including two (dipole illumination), four (quadrupole illumination), eight (octapole illumination), etc. The illumination mode used is very much dependent on the type of pattern being formed. For example, in the case of a line-and-space pattern, where offset of the illumination along only a single axis, e.g., x-axis, is needed, a dipole illumination shape can be used. For patterns more complex than line-and-space, where more than one axis of offset is needed, annular illumination can be used.
FIG. 5 is a schematic diagram of a typical semiconductor memory integrated circuit 150. The typical memory circuit 150 of FIG. 5 includes a memory cell region 151 and an adjacent peripheral circuit region 152 formed on a single circuit chip die. Generally, the memory cell region 151 has periodic line-and-space patterns, and the peripheral circuit region 152 has many unique patterns. Typically, when the memory circuit 150 is fabricated, exposure conditions are optimized for the circuits formed in the memory cell region 151. Therefore, the illumination mode used to fabricate the circuit 150 is selected according to the types of patterns in the memory cell region 151.
However, the illumination mode that is optimal for the memory cell region 151 typically will not be optimal for the peripheral circuit region 152, because the pattern types formed in the peripheral circuit region 152 are usually different and more complex than those formed in the memory cell region 151. For example, the memory cell region 151 is typically made up of periodic line-and-space patterns. Accordingly, a dipole illumination mode 153 is typically used for the exposure in the memory cell region 151. However, because the peripheral circuit region 152 includes more complex non-periodic circuitry of various shapes and sizes, dipole illumination is not optimal for the illumination in the region 152. Annular illumination 154 is more desirable for the peripheral circuit region 152. Therefore, if the same illumination type, e.g., dipole, is used for both regions 151 and 152, the resolution in region 152 will not be optimized.
In response to this, two illumination types can be used in exposing the memory cell region 151 and the peripheral circuit region 152. Specifically, dipole illumination 153 is used in exposing the memory cell region 151, and annular illumination 154 is used in exposing the peripheral circuit region 152. This is commonly accomplished by one of two possible approaches.
In a first approach, a two-step or double-exposure process is used. Under this approach, the memory cell region 151 is exposed first using dipole illumination 153. Then, in a second exposure step, the peripheral circuit region 152 is exposed using annular illumination 154. The order of the steps can be reversed. This approach has several drawbacks. It is time consuming and therefore inefficient and, therefore, suffers from low throughput. Also, it is difficult to overlay the regions and to stitch the regions together. As a result, this approach is not applicable to mass production applications.
Another approach uses a single exposure step which provides different illumination types simultaneously to the memory cell region 151 and the peripheral circuit region 152. FIG. 6 is a schematic diagram illustrating this approach. A shaped illumination from a source 12 is directed onto the photomask 226 having a pattern of opaque lines 227 made of an opaque material such as chrome formed on its bottom or back surface. The shaped illumination is typically of the annular type 154. The photomask 226 includes two regions 51 and 52. The illumination for the memory cell region 151 of the integrated circuit passes through the region 51 of the photomask 226, and the illumination of the peripheral circuit region 152 passes through the region 52 of the photomask 226. The photomask is fabricated to include a grating pattern 143 formed on the top surface of the photomask 226 in the region 51. The grating pattern 143 is formed to transform the annular illumination type 154 incident on the grating pattern 143 into dipole illumination type 153. As a result, the memory cell region 151 is illuminated with dipole illumination 153 and the peripheral circuit region 152 is simultaneously illuminated with annular illumination 154.
This approach also has several drawbacks. For example, the diffracted light passing through the back side of the photomask 226 under different illumination conditions overlaps at the boundary between the patterns for the two regions 51 and 52 at the back side of the photomask, as indicated by the dashed circle labeled “A” in FIG. 6. This results in an undesirable illumination condition at the boundary between the memory cell region 151 and the peripheral circuit region 152 on the integrated circuit. Generally, the diffracted light that passes through the back side of the photomask 226 is widely dispersed, resulting in low-quality pattern imaging and loss of spatial resolution.